Presently, two and three bed pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) processes are commonly used for production of oxygen. As the plant capacity increases, the total waste gas increases due to modest O2 recovery, and the vacuum pump(s) requires more time to remove the total waste during bed regeneration as pumps are operating closer to maximum throughputs. In such situations greater than 60% O2 recovery can be achieved using four or more beds to run the PSA process with additional steps to allow more time for bed regeneration and the recovery of void gases in the bed prior to counter-current blowdown. Compared to two and three bed processes, the four or more bed processes offer more degrees of freedom (process operating conditions, bed size and cycles) and lower unit costs.
Two-bed VPSA processes for the production of oxygen from air are disclosed by Baksh et al. (U.S. Pat. No. 5,518,526) and Smolarek et al. (U.S. Pat. No. 6,010,555). Both of these patents disclosed two-bed VPSA processes for producing O2 from air.
Smolarek et al. (U.S. Pat. No. 5,656,068) disclosed a four-bed VPSA process, operated as two pairs of 2-bed systems, referred to as 2×2 cycle/system, to produce oxygen from air. Each pair of beds is operated 180° out of phase and the two pairs of beds are operated out of phase by one-half of a half-cycle. Two compressors (one Roots or positive displacement and one centrifugal) and two vacuum pumps (one Roots or positive displacement and one centrifugal) are utilized in the VPSA process of U.S. Pat. No. 5,656,068. One of the two compressors is periodically in the idle or vent mode.
Doong (U.S. Pat. No. 5,997,612) disclosed a four-bed VPSA system operating as two pairs of adsorption beds to produce about 100 tons per day (TPD) oxygen. The VPSA process includes two pairs of beds, an intermediate storage tank (to collect co-current depressurization for use in purging of the beds), one gas blower and a pair of vacuum pumps. The system disclosed in U.S. Pat. No. 5,997,612 utilizes three pumps versus the four pumps used in U.S. Pat. No. 5,656,068. Furthermore, the system disclosed in U.S. Pat. No. 5,997,612 handles a single feed stream versus a dual feed stream as disclosed in U.S. Pat. No. 5,656,068.
More recently, a four-bed VPSA system and process with a dual feed inlet for large scale oxygen production (e.g., >350 tons/day O2) has been disclosed in U.S. patent application Ser. No. 11/264,278. The system disclosed uses two compressors and one vacuum pump rather than the two compressors and two vacuum pumps disclosed in U.S. Pat. No. 5,656,068.
In the application of VPSA or PSA processes, the energy input required to achieve the separation of O2 from the feed mixture (e.g., air) is provided as mechanical work through feed compressor(s) and vacuum pump(s). The cost of this work is a significant component of the total operating cost of the VPSA process. Also, VPSA or PSA is currently economically competitive with cryogenic distillation for small scale applications only. In order for PSA or VPSA processes to become cost competitive with cryogenic distillation for large scale applications, four or more beds are required to operate improved PSA cycles to achieve enhanced PSA process performance and to lower operating cost.
Therefore, it would be desirable to provide an improved multiple bed system that could provide enhanced performance and lower operating cost for large scale oxygen production.